There have been proposals made for hybrid vehicles including a motor generator besides an internal combustion engine (referred herein sometimes simply as an engine) for traction use. In those hybrid vehicles, the motor generator has permissible output torque that can be output such as when driving or when generating electricity, as it is determined by inherent characteristics. The permissible output torque is substantially constant within a predetermined revolution number range (as a rotation speed range) constituting a normal service range of the motor generator.
When the internal combustion engine has a varied output characteristic due to disturbances such as variations in the environment, such as those in the temperature of suction air or in the atmospheric pressure, for instance, the motor generator being driven to rotate may have a revolution number exceeding a specific upper limit of revolution number, reaching a high revolution number range, entering an over-speed state. Like this, when the motor generator is over-speeded, the permissible output torque of the motor generator tends to more approach to a zero than a constant value in the normal service range. That is, the motor generator has a decreased absolute value of torque. If the motor generator is given a command for an absolute value of torque to be smaller than the absolute value of its permissible output torque, it can produce a value of torque matching the command torque. However, if the motor generator is given a command for an absolute value of torque to be larger than the absolute value of its permissible output torque, it fails to produce any value of torque matching the command torque. There is torque produced at the motor generator, which is limited to the permissible output torque or smaller, so the produced torque becomes extremely limited as the permissible output torque gets smaller in the absolute value, approaching a zero. Such being the case, there were situations involving a motor generator failing to produce necessary torque, causing a failed control in a driving system including the motor generator, as an inconvenience constituting an obstacle to the running of a vehicle.
There are known technologies for controlling output torque of an internal combustion engine to prevent such over speeds of a motor generator (refer to the PTL 1 and PTL 2 below, for instance). The PTL 1 has disclosed a technology of making a control based on a maximum output correction value set to a maximum output of a motor generator, giving an allowance value, which permits a prevention to be achieved against over speeds of the motor generator. The PTL 2 has disclosed a technology for a correction to be made, when an engine control unit (ECU) has determined an abnormality in communications with a control unit for hybrid use, in the manner of decreasing a target revolution number in a revolution number feedback control for an internal combustion engine, in accordance with output torque of the internal combustion engine. In other words, according to the technology disclosed in the PTL 2, the internal combustion engine has a revolution number controlled to keep a motor generator from over-speeding.